Try again if mktime() fails when we thought we knew DST-ness

When refreshing a QDateTime(,, Qt::LocalTime) we call mktime on data
obtained from it, passing in the DST status (when known; this keeps
two otherwise identical times in a fall-back distinct). One of the
tests relies on changing zone under the feet of such a date-time,
created in Hawaiian standard time; it serializes it, the reads it back
in Western Australian Daylight-saving time and expects the results to
be equal. However, the two differ in DST-ness, which leads to mktime()
failing for the Hawaiian original, with unwelcome results.

Notice this case, failure with DST-ness claimed known, and retry
without the claim, so as to correct the DST-ness.

Change-Id: Id0278df53130f76fc587769efe946ca9af1adc26
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -2457,6 +2457,12 @@ static qint64 qt_mktime(QDate *date, QTime *time, QDateTimePrivate::DaylightStat
     int hh = local.tm_hour;
 #endif // Q_OS_WIN
     time_t secsSinceEpoch = qMkTime(&local);
+    // That can fail if we thought we knew DST-ness, but were wrong:
+    if (secsSinceEpoch == time_t(-1) && local.tm_isdst >= 0) {
+        local.tm_isdst = -1;
+        secsSinceEpoch = qMkTime(&local);
+    }
+
     if (secsSinceEpoch != time_t(-1)) {
         *date = QDate(local.tm_year + 1900, local.tm_mon + 1, local.tm_mday);
         *time = QTime(local.tm_hour, local.tm_min, local.tm_sec, msec);